An origin of North American monsoon retreat biases in climate models

Ye, Jiacheng

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https://hdl.handle.net/2142/116125 Copy

Description

Title

An origin of North American monsoon retreat biases in climate models

Author(s)

Ye, Jiacheng

Issue Date

2022-07-21

Director of Research (if dissertation) or Advisor (if thesis)

Wang, Zhuo

Committee Member(s)

• Yang, Fanglin
• Dominguez, Francina

Department of Study

Atmospheric Sciences

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• North American monsoon
• Climate modeling

Abstract

Most state-of-the-art coupled climate models suffer from a late retreat bias in the North American monsoon (NAM) simulations, which is manifested by overestimated precipitation in October. The overestimated precipitation has long been attributed to the negative sea surface temperature (SST) biases in the tropical Atlantic and insufficient model resolution to resolve mesoscale features, including complex terrains, local land-sea contrast, and Gulf of California (GoC) surges. However, we found little correlation between CMIP6 model resolutions and the simulated NAM retreat-season precipitation. Instead, we showed that tropical eastern North Pacific SST biases and the associated large-scale circulation biases play a dominant role in inducing the retreat-season biases, with SST biases in other ocean basins (i.e., the subtropical eastern Pacific and the tropical North Atlantic) playing a secondary role. As revealed by simulations using a hierarchy of models, the positive SST biases in the tropical eastern North Pacific enhance local convection and lead to positive diabatic heating biases locally throughout the troposphere; the diabatic heating biases generate a Matsuno-Gill type response that strengthens the subtropical high over the North Atlantic and weakens the subtropical high over the North Pacific, enhancing the low-level northward moisture transport from the tropics to the NAM region. The conclusion is robust across CMIP6 models. The precipitation seasonality in the NAM region is also used to constrain future projection. The “good” CMIP6 models project a drier monsoon peak season and a delayed monsoon retreat while the “poor” CMIP6 models project enhanced monsoon precipitation. All model groups project a drier dry season in the NAM region.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Jiacheng Ye

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